1. Field of the Invention
The invention relates to a developing apparatus for use in a copying machine, a printer, a facsimile apparatus or the like using an electrophotographic process or an electrostatic recording process.
2. Description of Related Art
Heretofore, in multi-color image forming, use has been made of a technique of forming a plurality of colors of toner images successively, for example, on an electrophotographic photosensitive member (photosensitive member) which is an image bearing member, and successively or collectively superimposing the plurality of colors of toner images onto a transfer medium such as recording paper.
In such a multi-color image forming apparatus, there has heretofore been proposed and put into practical use a so-called rotary developing process of mounting developing devices for black, yellow, magenta and cyan, for example, on a rotary type developing member, and rotating this rotary type developing member to thereby successively move the necessary developing devices to a developing position opposed to the photosensitive member which is the image bearing member and perform a developing operation.
On the other hand, in image forming apparatuses of a conventional electrophotographic type or electrostatic recording type, above all, particularly in a multi-color image forming apparatus for effecting colored image forming, use has widely been made of a two-component developing process using chiefly a mixture of a nonmagnetic toner and a magnetic carrier as a developer. The two-component developing process, as compared with other developing processes being proposed at present, has merits such as the stability of image quality and the durability of the apparatus. On the other hand, in the two-component developing process, the deterioration of the developer due to a long period of endurance, particularly the deterioration of the carrier, has been unavoidable and therefore, with the long period of use of the apparatus, the work of interchanging the developer has become necessary, and this has brought about an increase in service cost and running cost.
Several methods of solving such a problem have heretofore been proposed. For example, there is a developing device loaded with an interchangeable developer supplying cartridge to thereby effect the supply of a developer including a toner and a carrier, and the collection of the developer (see Japanese Patent Application Laid-Open No. 6-308829).
In this developing device, however, there is adopted structure using a plurality of screws to collect the developer overflowing from the developing device into the developer supplying cartridge and therefore, the apparatus becomes large-scaled, and a control mechanism also becomes complicated. Also, when the developing device of such a construction and the developer supplying cartridge are carried on a rotary type developing member, the diameter of the rotary type developing member becomes large and the apparatus becomes bulky. Further, a developer conveying route is complicated, and when the rotary type developing member is rotated, the developer may leak.
So, particularly in the rotary developing process, there has been put into practical use a developer discharging process of utilizing a change in the acting direction of gravity resulting from the rotational motion of the rotary type developing member to supply a two-component developer to the developing device, and discharge the two-component developer from the developing device.
For example, Japanese Patent Application Laid-Open No. 9-218575 discloses a rotary developing process designed such that as shown in FIG. 6 of the accompanying drawings, a developer including an amount of toner consumed by a developing operation at a developing position P1 opposed to a photosensitive drum 201 is supplied to a developing device 210, and any excessive developer in the developing device 210 is discharged to a developer cartridge 220 through a discharge port 221, a transport tube 222 and a collection port 223 by the utilization of a change in the acting direction of gravity by the rotation of a rotary type developing member 204. That is, this process is simple in structure because movement peculiar to the rotary type developing member 204 is utilized to effect the supply and collection of the developer, and a reduction in charging capability for a carrier is prevented without causing the bulkiness and a rise in the cost of the apparatus.
On the other hand, Japanese Patent Application Laid-Open No. 10-142888 discloses a rotary developing process designed such that as shown in FIG. 7 of the accompanying drawings, a developer discharged from a developing device 310 through a discharge port 321 at a developing position P1 whereat the developing device 310 is opposed to a photosensitive drum 301 is temporarily stored in a storing portion 322, and by the utilization of a change in the acting direction of gravity by the rotation of a rotary type developing member 304, the developer in the storing portion 322 is transported to a cylindrical shaft 323 at the center of the rotary type developing member 304 through a collection port 323a, and is finally collected into a developer collecting container (not shown) at an end portion of the cylindrical shaft 323 by a developer transporting member 323b in the cylindrical shaft 323. That is, as in an image forming apparatus proposed in Japanese Patent Application Laid-Open No. 9-218575, movement peculiar to the rotary type developing member 304 is utilized to effect the discharge of the developer. Therefore, a reduction in charging capability for a carrier is prevented without causing the bulkiness of the apparatus. In addition, design is made such that even when single-color image forming is continued, the developing operation is not stopped, but at the developing position, the excessive developer in the developing device 310 is discharged to the storing portion 322 outside the developing device 310 and therefore, the lowering of image productivity is suppressed to a certain extent, and the amount of developer in the developing device 310 is maintained within an allowable value.
The above-described examples of the conventional art, however, have suffered from the following problems.
In the midst of recent years when the market of full-color copying machines/printers has been enlarged and various functions are required, there have been commercialized many image forming apparatuses having been downsized and reduced in cost, and yet seeking for high image productivity, and this seems to become one of the mainstreams of the market in the future, too.
In the image forming apparatus described in the above-mentioned Japanese Patent Application Laid-Open No. 9-218575, the rotational motion of the rotary type developing member 204 is utilized to discharge the deteriorated developer to the developer cartridge and therefore, structure is simple and the bulkiness and high cost of the apparatus are not caused and the deteriorated developer is gradually replaced with a fresh developer, to thereby stabilize the characteristic as the whole of the developer. Therefore, the interchange of the developer becomes unnecessary and maintenance property is improved. However, at the developing position P1, the excessive developer in the developing device 210 is not discharged to the outside of the developing device 210 and therefore, particularly when single-color high density images are continuously formed, in order to prevent any increase in the amount of developer in the developing device 210, the developing operation must be stopped in spite of images being formed and the operation of discharging the developer, i.e., the operation of rotating the rotary type developing member 204, must be performed, and a great reduction in image productivity could not be avoided.
On the other hand, in an image forming apparatus described in the above-mentioned Japanese Patent Application Laid-Open No. 10-142888, the deteriorated developer is discharged into the collecting container provided at an end portion of the cylindrical shaft 323 of the rotary type developing member 304 and is gradually replaced with a fresh developer to thereby stabilize the characteristic as the whole of the developer. Thus, the interchange of the developer becomes unnecessary and maintenance property is improved and in addition, design is made such that at the developing position P1, the excessive developer in the developing device 310 is discharged to the storing portion 322 outside the developing device 310 and therefore, during continuous single-color image forming, the developer can be stably discharged without the rotary type developing member 304 being rotated up to a certain degree of amount of excessive developer discharge. Accordingly, within the allowable range thereof, image productivity is not lowered due to the developer discharging operation.
Again in this image forming apparatus, however, as in the image forming apparatus described in the above-mentioned Japanese Patent Application Laid-Open No. 9-218575, basically a change in the direction of gravity by the rotative movement of the rotary type developing member 304 is utilized to discharge the developer to the collection port 323a, and make the developer fall and therefore, when the discharged developer is too much stored in the storing portion 322 (exceeds the aforementioned allowable range), the discharging operation cannot be performed any further.
Therefore, particularly when single-color high-density images are continuously formed, in order to prevent any increase in the amount of developer in the storing portion 322, the developing operation must be stopped in spite of images being formed, and the operation of discharging the developer, i.e., the operation of rotating the rotary type developing member 304, must be performed. Thus, a great reduction in image productivity could not be avoided.
Also, the storing portion 322 must be discretely provided in the developing device 310 and therefore, a plurality of members unnecessary for ordinary image forming must be provided, and the complication and a rise in the cost of the apparatus could not be avoided.
Now, heretofore, in image forming apparatuses of an electrophotographic type having a rotary type developing member, there has often been seen a construction in which a developer bearing member provided in a developing device is made to wait at a phase position (so-called home position) in which it is not opposed to a photosensitive member so that when an image forming operation is not being performed, the photosensitive member may not be adversely affected by the strong magnetic force of the developer bearing member provided in the developing device.
Also, when there is adopted a construction in which a developer cartridge comprising a toner bottle is detachably mounted on the rotary type developing member, there is a case where the phase position at which the developer bearing member is stopped and waits in order to interchange the developer cartridge is provided at a phase position discrete from the above-described developing position or home position stop and waiting position.
When such a construction is adopted, besides a “development stopping position” which is the stop position of the rotary type developing member when a developing device for each color is disposed at the developing position, the stop and waiting positions of a plurality of rotary type developing members become necessary. For example, the stop positions of developing device provided in a rotary type developing member holding developing devices for four colors are not restricted to four locations of 90 degrees, 180 degrees, 270 degrees and 360 degrees from the developing position which is at 0 degree, but cannot help having a plurality of stop positions besides them.
However, both of the image forming apparatuses described in the above-mentioned Japanese Patent Application Laid-Open No. 9-218575 and Japanese Patent Application Laid-Open No. 10-142888 are of a construction in which a change in the direction of gravity by the rotative movement of the rotary type developing member is utilized to make the excessive developer fall to the destination of discharge and therefore, as the rotation stopping positions are increased, a discharging mechanism becomes more unstable, whereby there are cases where unexpected discharge is effected, and it becomes difficult to maintain a high quality of image.
Particularly, in the image forming apparatuses having a rotary type developing member, the shortening of a changeover time for this rotary type developing member has been advanced in order to cope with the high productivity in recent years. In this case, the shock force when the rotation of the rotary type developing member is started and stopped tends to become great, but in other rotation modes than a continuous image forming operation, it is often the case that an acceleration and deceleration curve is set to a slow one in order to make the shock force small. When as described above, the image forming apparatus has a rotation mode for a plurality of rotary type developing members, the instability of a discharging mechanism resulting from the above-described increase in the rotation stopping positions tends to be further promoted.
This is because during a continuous image forming operation, the aforedescribed development stopping position visits periodically and stably, whereas the “home position waiting position” is for stopping and waiting only after the termination of a job and the “developer cartridge interchange stopping and waiting position” is for stopping and waiting only when the toner in the cartridge has become little. If an attempt is made to include all these conditions and obtain a stable discharge characteristic of the developer, it has required much difficulty in setting the conditions.
That is, in the conventional art, the work of interchanging the developer becomes unnecessary and maintenance property is improved, but it is difficult to obtain a stable high image quality maintaining characteristic, and particularly taking into account a case where single-color high-density images continue or the like, it has been very difficult to realize all of the simplification and reduced cost of the apparatus and high image productivity at a time.